1. Field of the Invention
The present invention relates generally to offshore drilling operations, and more specifically to an apparatus for removing and installing drilling equipment.
2. Background of the Invention
An offshore drilling facility includes a drilling rig for various operations performed while drilling a subsea well. A conductor casing extends from the subsea well to an elevation above the surface of the water, where a working platform is connected to an upper portion of the conductor casing for drilling operators to work. On many facilities, the drilling rig is often supported by a drilling platform or cantilever that extends from a side of the drilling facility, over the water, to a location above the drilling platform and conductor casing.
A string of drill pipe is lowered from the drilling rig, through an opening in the cantilever, into the conductor casing for conducting drilling operations. A blow out preventer (BOP), typically located within the cantilever, engages the upper end of the conductor casing. The BOP regulates fluid flow out of the conductor casing from the subsea well. There are numerous types of BOPs. For example, some have rams that clamp down on the conductor casing. Others are annular which contract to enclose around a drill sting. The term “BOP” as used in this application is not meant to designate either device, but rather to encompass each type known or foreseeable in the art. BOPs can weigh up to 40,000 lbs., and have large pieces that are cumbersome while being moved between various positions.
During some operations, the BOP must be removed. During these operations, the BOP is typically hoisted and slid along rails to another location within the cantilever. Should a BOP need replacing or to be removed from the drilling facility, the upper surface of the cantilever prevents a crane from lifting the BOP from the storage or stowed position. Rather, the BOP must be moved to a location below the opening the drill pipe typically extends through for engaging the conductor casing.
The drilling rig cannot lift the BOP and continue drilling operations through the same opening in the upper surface of the cantilever. Therefore, the drilling rig discontinues drilling operations and disconnects from the conductor casing to lift the BOP out of the cantilever. Accordingly, the cantilever and drilling rig must be lifted, or “jacked up”, relative to the conductor casing in order to disconnect from the conductor casing and allow room for the BOP over the conductor casing. In order to lower the BOP onto a vessel, the drilling rig is then slid on the cantilever so that the BOP is at a position that is not directly above the conductor casing. Because the BOP is being lowered from the drilling rig, the vessel is in close proximity to the conductor casing and the supports of the facility. This creates a dangerous environment for the workers and the facilities. During normal weather conditions, this removal process can take two to three days. Under rough weather conditions, the process can take four to six days.